As this sort of exhaust gas dilution device, as disclosed in JPA 2000-329661, as a device that measures particulate matter (PM) contained in exhaust gas discharged from an engine, there is one that has a dilution tunnel, an exhaust gas introduction pipe for introducing the exhaust gas into the dilution tunnel, and an orifice plate that is provided near an introduction port of the exhaust gas introduction pipe and formed with an orifice.
Also, as disclosed in International Publication No. WO2010/112286, there is an exhaust gas dilution device that is, by providing a single dilution tunnel with a first exhaust gas introduction pipe for introducing first exhaust gas and a second exhaust gas introduction pipe for introducing second exhaust gas, and switching between the exhaust gas introduction pipes by a switching valve, configured to be able to selectively dilute and measure any of the exhaust gases from respective engines. In addition, in the exhaust gas dilution device, on a downstream side of an orifice that mixes the first exhaust gas and dilution air with each other, the second exhaust gas introduction pipe is connected.
However, the second exhaust gas introduction pipe is connected on the downstream side of the orifice, and therefore in the case of performing PM measurement of the first exhaust gas, there is a problem that the first exhaust gas diluted by the orifice is retained in opening space of the second exhaust gas introduction pipe or attached on an inner surface forming the opening space to cause a measurement error in the PM measurement of the first exhaust gas. That is, on the downstream side of a mixing part where the first exhaust gas and the dilution gas are mixed, the exhaust gas is cooled by the dilution gas to produce PM. Further, in the case of providing the dead space on the downstream side of such a mixing part, the PM is attached in the dead space to cause the measurement error. Also, in the case of performing PM measurement of the second exhaust gas after the end of the PM measurement of the first exhaust gas, there is a problem that the PM that arises from the first exhaust and is retained or attached in the opening space of the second exhaust gas introduction pipe may be measured as PM contained in the second exhaust gas, and therefore a measurement error occurs in the PM measurement of the second exhaust gas.